1. Field of the Disclosure
This invention pertains to a method for preparing a relief printing form from a photosensitive element, and in particular, to a method of preparing the relief printing form by imagewise exposing the photosensitive element in a specific environment, and then thermally treating.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials ranging from corrugated carton boxes to cardboard boxes and to continuous web of plastic films. Flexographic printing plates are used in relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. Photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. Flexographic printing plates are characterized by their ability to crosslink or cure upon exposure to actinic radiation. The plate is imagewise exposed with actinic radiation through an image-bearing art-work or a template, such as a photographic negative or transparency (e.g., silver halide films) for so called analog workflow, or through an in-situ mask having radiation opaque areas that had been previously formed above the photopolymerizable layer for so called digital workflow. The actinic radiation exposure is typically conducted with ultraviolet (UV) radiation. The actinic radiation enters the photosensitive element through the clear areas and is blocked from entering the black or opaque areas of the transparency or in-situ mask. The areas of the photopolymerizable layer that were exposed to the actinic radiation crosslink and harden and/or become insoluble to solvents used during development. The unexposed areas of the photopolymerizable layer that were under the opaque regions of the transparency or the in-situ mask during exposure do not hardened and/or remain soluble. The unexposed areas are removed by treating with a washout solution or heat leaving a relief image suitable for printing. If treated with washout solution, the plate is dried. After all desired processing steps, the plate is then mounted on a cylinder and used for printing.
An alternative to solution development, the thermal development process is desired as the process removes the unexposed areas while avoiding the subsequent time-consuming drying step associated with solution development. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. No. 3,060,023 (Burg et al.); U.S. Pat. No. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson at al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated from the photosensitive layer. The cycle of heating and contacting the photosensitive layer with the absorbent material may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. After such processing, there remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
However with increasing demands on quality, the current state-of-the-art flexographic printing forms may not perform as desired and have trouble meeting the ever increasing demands on quality.
Solid plate preparation workflows involve making an intermediate, i.e., the photographic negative or phototool. Analog workflow requires the preparation of the phototool, which is a complicated, costly and time-consuming process requiring separate processing equipment and chemical development solutions. In addition, the phototool may change slightly in dimension due to changes in temperature and humidity. The same phototool, when used at different times or in different environments, may give different results. Use of a phototool also requires special care and handling when fabricating flexographic printing forms to ensure intimate contact is maintained between the phototool and plate. In particular, care is required in the placement of both the phototool and the plate in the exposure apparatus along with special materials to minimize air entrapment during creation of a vacuum to ensure intimate contact. Additionally care must be taken to ensure all surfaces of the photopolymer plate and phototool are clean and free of dust and dirt. Presence of such foreign matter can cause lack of intimate contact between the phototool and plate as well as image artifacts.
An alternative to analog workflow is termed digital workflow, which does not require the preparation of a separate phototool. Photosensitive elements suitable for use as the precursor and processes capable of forming an in-situ mask in digital workflow are described in U.S. Pat. Nos. 5,262,275; U.S. 5,719,009; U.S. 5,607,814; U.S. 6,238,837; U.S. 6,558,876; U.S. 6,929,898; U.S. 6,673,509; U.S. 5,607,814; U.S. 6,037,102; and U.S. 6,284,431. The precursor or an assemblage with the precursor includes a layer sensitive to laser radiation, typically infrared laser radiation, and opaque to actinic radiation. The infrared-sensitive layer is imagewise exposed with laser radiation whereby the infrared-sensitive material is removed from, or transferred onto/from a superposed film of the assemblage, to form the in-situ mask having radiation opaque areas and clear areas adjacent the photopolymerizable layer. The precursor is exposed through the in-situ mask to actinic radiation in the presence of atmospheric oxygen (since no vacuum is needed). Furthermore, due in part to the presence of atmospheric oxygen during imagewise exposure the flexographic printing form has a relief structure that is different from the relief structure formed in analog workflow (based upon the same size mask openings in both workflows). Digital workflow creates a raised element (i.e., dot) in the relief structure having a surface area of its uppermost surface (i.e., printing surface) that is significantly less than the opening in the in-situ mask corresponding to the relief structure. Digital workflow results in the relief image having a different structure for dots (i.e., raised surface elements) that is typically smaller, with a rounded top, and a curved sidewall profile, often referred to as dot sharpening effect. Dots produced by analog workflow are typically conical and have a flat-top. The relief structure formed by digital workflow results in positive printing properties such as, finer printed highlight dots fading into white, increased range of printable tones, and sharp linework. As such, the digital workflow because of its ease of use and desirable print performance has gained wide acceptance as a desired method by which to produce the flexographic printing form. But not all end-use applications view this dot-sharpening effect as beneficial.
It is known by those skilled in the art that the presence of oxygen (O2) during exposure in free-radical photopolymerization processes will induce a side reaction in which the free radical molecules react with the oxygen, while the primary reaction between reactive monomer molecules occurs. This side reaction is known as inhibition (i.e., oxygen inhibition) as it slows down the polymerization or formation of crosslinked molecules. Many prior disclosures acknowledge that it is desirable for photopolymerization exposure to actinic radiation to occur in air (as is the case for digital workflow), under vacuum (as is the case for analog workflow), or in an inert environment. Oftentimes, nitrogen is mentioned as a suitable inert gas for the inert environment. The implication is that the nitrogen environment is one that contains substantially less than atmospheric oxygen to the extent that all oxygen is removed, or something less than about 10 ppm of oxygen. Nitrogen with oxygen impurity concentration level less than 10 ppm is readily commercially available.
The effect of oxygen associated with digital workflow can impact the ability to hold solid screen patterns in solid printing areas of the relief printing form. It is often desirable for an image that is printed by flexographic relief printing form to increase the density of ink in solid areas of the image, so-called solid ink density. Solid screening is a well-known process for improving the solid ink density in flexographic printing, and is described for instance in U.S. Pat. No. 6,492,095; and U.S. Patent Publication US 2010/0143841. Solid screening consists of creating a pattern in the solid printing areas which is small enough that the pattern is not reproduced in the printing process (i.e., printed image), and large enough that the pattern is substantially different from the normal, i.e., unscreened, printing surface. Often these screening patterns are features in the range of 5 to 30 microns. The inhibition effect of the atmospheric oxygen during imagewise exposure of a photosensitive element for relief printing can result in pattern features being reduced in size by about 15 microns on each edge. Consequently this reduction in feature size, a 30 micron feature will be reduced to a 0 micron feature size for example, limits relief printing form to print increased solid ink densities.
There is also a desire to transition from solvent development to thermal development in the fabrication of solid plates, due to the environmental and economic advantages of thermal development. However, a problem sometimes arises with thermal processing in that the uncured photopolymer is not always adequately cleaned out or removed from recessed areas of the relief surface of the printing element. Inadequate clean out of recessed areas can manifest as insufficient removal of photopolymer material to the relief depth desired and/or as a non-uniformity of relief depth between open floor areas and in relatively narrow channels or gaps between large raised areas (i.e., typically solid printing areas). Relief depth is the difference between thickness of a floor of cured polymer and the thickness of the printing layer in the printing element. While in general it is particularly difficult to adequately clean out or remove uncured photopolymer material in relief printing forms in which the photopolymerizable layer has a thickness greater than about 100 mil, it can be difficult to remove uncured material from recessed areas of the relief surface even for relief printing forms in which the photopolymerizable layer has a thickness less than 100 mil. In particular, relief printing forms that are prepared from photosensitive elements using analog workflow and thermal development can exhibit inadequate clean out.
In general, relief printing forms that are prepared from photosensitive elements using digital workflow and solvent development exhibit suitable relief depth and uniformity in open floor areas and narrow channels or gaps between solid printing areas, and do not encounter the problem of inadequate clean out or removal from recessed areas of the relief surface.
Relief printing forms having incomplete clean out or removal of recessed areas of the relief surface can result in poor print performance, that is, poor reproduction of the image printed on the substrate. Relief printing is a method of printing in which the printing form prints from an image area, where the image area of the printing form is raised and the non-image area is depressed or recessed. Recessed areas, such as the floor, that are not cleaned out sufficiently are shallow, and thus can pick up ink and contact to transfer the ink onto the substrate in regions that are not to be printed. This is sometimes referred to as “printing the floor”. In other instances, small dirt particles or lint can cling to the shallow recessed areas, pickup ink, and transfer ink (with or without the dirt particle) to the substrate, which can render the printed image “dirty”. This effect of printing shallow floors or dirt can be exacerbated since the printing form is often in pressure contact with the substrate.
There is also a desire to eliminate the costs and the time consuming and problematic process steps associated with the preparation of the photographic negative intermediate, and transition from analog workflow to digital workflow in the fabrication of solid plates. However, the dot-sharpening effect associated with conventional digital exposure in the presence of atmospheric oxygen becomes a disadvantage for printing forms that are thermally treated to form the relief. Because imagewise exposure in digital workflow is conducted in the presence of atmospheric oxygen which inhibits polymerization, the dot structure of the raised surface element has a printable surface area that is considerably reduced (compared to that produced by analog workflow, as well as compared to the corresponding opening in the digital mask image). Moreover, fine raised surface elements associated with printing of the highlights of an image, for example, raised surface elements representing less than or equal to about 3% dots, may have insufficient height above the floor, may be warped, or may be bent such that the printing result is deemed to be deficient. As such, it is desirable to fabricate a relief printing form using digital workflow due to its ease and simplicity, while providing a relief surface in the printing form that is capable of printing detailed fine highlights in halftone images and/or clean fine text and line graphics. Further, it is desirable to fabricate a relief printing form by thermal processing instead of solvent processing, but to attain thermally-developed printing forms having relief structure of images and graphics that is comparable or substantially comparable to the relief structure of printing forms prepared by solvent processing.
Thus, the need to use digital workflow and thermal development in the fabrication of relief printing forms conflicts with the need for the printing form to have a relief surface that is capable of printing desired images and graphics with clean open areas, fine text and line graphics, as well as the full range of halftone images from dark shadows to fine highlights. The relief printing form should also be capable of holding solid screen patterning in solid areas for printing solid ink areas with increased density. So a need arises for a method of fabricating a relief printing form from a photosensitive precursor element that utilizes digital workflow to eliminate the costly and problematic creation of a separate intermediate, and that utilizes thermal development to reduce the time to prepare the printing form. The relief printing form needs to have a relief structure composed of recessed areas and raised areas for forming a pattern of printing regions on a substrate that contain detailed fine highlights in halftone images, clean open areas between raised areas, and/or clean fine text and line graphics.